There is a continuing need in the petroleum industry for improved catalyst supports and supported catalysts derived therefrom, which have enhanced activity and improved catalyst life and exhibiting a desirable balance of morphological properties.
Foraminous carriers in particulate form have been found to be useful for example as catalyst supports and in preparing catalysts for chemical processes. Such applications include added catalytic materials, such as metallic ions, finely-divided metals, cations, and the like, to the carrier. The level and distribution of these metals on the support, as well as the properties of the support itself are representative parameters that influence the complex nature of catalytic activity and life.
For supported catalysts used in chemical reactions, the morphological properties of the support, such as surface area, pore volume, pore size and pore size distribution of the pores that comprise the total pore volume are very important. Such properties influence the nature and concentration of active catalytic sites, the diffusion of the reactants to the active catalyst site, the diffusion of products from the active sites and catalyst life. In addition, the support and its dimensions also influence the mechanical strength, density and reactor packing characteristics, all of which are important in commercial applications.
Hydroprocessing catalysts in petroleum refining represent a large segment of alumina-supported catalysts in commercial use and such hydroprocessing applications span a wide range of feed types and operating conditions, but have one or more common objectives, namely, removal of heteroatom impurities (such as components selected from the group consisting of sulfur-containing compounds, nitrogen-containing compounds, metal-containing compounds (sometimes referred to as sulfur, nitrogen and metals), asphaltenes, carbon residue, sediment precursors, and mixtures thereof), increasing the hydrogen to carbon (H/C) ratio in the products (thereby reducing aromatics, density and/or carbon residues), and cracking carbon bonds to reduce boiling range and average molecular weight.
As refiners increase the proportion of heavier, poorer quality crude oil in the feedstock to be processed, the need grows for processes to treat the fractions containing increasingly higher levels of metals, asphaltenes, and sulfur. It is widely known that various organometallic compounds and asphaltenes are present in petroleum crude oils and other heavy petroleum hydrocarbon streams, such as petroleum hydrocarbon residua, hydrocarbon streams derived from tar sands, and hydrocarbon streams derived from coals. The most common metals found in such hydrocarbon streams are nickel, vanadium, and iron. Such metals are very harmful to various petroleum refining operations, such as hydrocracking, hydrodesulfurization, and catalytic cracking. The metals and asphaltenes cause interstitial plugging of the catalyst bed and reduced catalyst life and metals present in such streams which deposit on hydroprocessing catalysts tend to poison or deactivate the catalyst. Moreover, asphaltenes tend to reduce the susceptibility of the hydrocarbons to desulfurization. If a catalyst, such as a desulfurization catalyst or a fluidized cracking catalyst, is exposed to a hydrocarbon fraction that contains metals and asphaltenes, the catalyst can become deactivated rapidly and thus be subject to premature replacement.
Various hydroconversion processes are effectively carried out using an ebullated bed (EB) system. In an EB, preheated hydrogen and resid feedstock enter the bottom of a reactor wherein the upward flow of resid with or without an liquid internal recycle suspend the catalyst particles in the liquid phase. In improved EB processes, part of the catalyst is continuously or intermittently removed in a series of cyclones and fresh catalyst is added to maintain activity. Approximately about 1 wt. % of the catalyst inventory is replaced each day in an ebullated bed system. Thus, the overall system activity is the weighted average activity of catalyst varying from fresh catalyst particles to old or substantially deactivated particles. More particularly, the use of a series of ebullated bed reactors containing a catalyst having improved effectiveness and activity maintenance in the desulfurization and demetallation of metal-containing heavy hydrocarbon streams are known.
In general, it has been desirable to design a hydroprocessing catalyst so that it exhibits the highest surface area in order to provide the maximum concentration of catalytic sites and activity. However, surface area and pore diameter are inversely related within practical limits. Consequently, a catalyst support, such as alumina particles, containing predominantly small pores will exhibit the highest surface area. In contrast, sufficiently large pores are required for diffusion of feedstock components, particularly as the catalyst ages and fouls, but larger pores have a lower surface area. More specifically, the catalyst formulator or designer as well as the process engineer is faced with competing considerations which often dictate a balance of morphological properties for supports as well as catalysts derived therefrom.
For example, it is recognized (see for example, U.S. Pat. No. 4,497,909) that while pores having a diameter below 60 Angstroms (Å), within the range of what is referred to therein as the micropore region, have the effect of increasing the number of active sites of certain silica/alumina hydrogenation catalysts, these very same sites are the first ones clogged by coke thereby causing a reduction in catalyst activity. Similarly, it is also accepted that when such catalysts have more than 10% of the total pore volume occupied by pores having a pore diameter greater than 600 Å, within the region referred to herein generally as the macropore region, the mechanical crush strength is lowered as is the catalyst activity. Finally, it is recognized, that for certain silica/alumina catalysts, that maximization of pores having a pore diameter between 150 Å and 600 Å, approximately within the region referred to therein as the mesopore region, is desirable for acceptable activity and catalyst life.
Thus, while increasing the surface area of the catalyst can increase the number of the active sites, such surface area increase naturally results in an increase of the proportion of pores in the micropore region and micropores are more easily clogged by coke. In short, increases in surface area and maximization of mesopore diameter are antagonistic properties. Moreover, not only must the surface area be high, but it should also remain stable when exposed to petroleum feedstock conversion conditions such as high temperature and moisture. There has therefore been a continuing search for stable carrier particles that exhibit a combination of pore size distribution and total surface area that can provide a combination of performance characteristics suitable for use as catalyst supports, particularly when used to support catalytically active metals for producing hydroprocessing catalysts.
It is further recognized that the physical and chemical properties of the carrier can depend on the procedures followed in its preparation and that many preparation processes have been developed in attempts to optimize its properties for use as a catalyst support material. Examples of suitable foraminous carrier materials are described hereinbelow. A carrier material such as alumina is frequently precipitated by combining a water-soluble, acidic aluminum compound which may be an aluminum salt such as aluminum sulfate, aluminum nitrate, or aluminum chloride, and an alkali metal aluminate such as sodium or potassium aluminate. (See for example, U.S. Pat. No. 4,154,812, Sanchez, M. G. and Laine, N. R., assigned to W. R. Grace & Co., which is incorporated herein to the extent permitted.) Thus, while catalyst carriers, including alumina carriers, are known, further improvements are needed in order to provide carriers having still further improved properties.
When used in ebullated bed resid hydrocracking processes, embodiments of the present invention increase 1000+° F. vacuum residuum (VR) or “resid” conversion and hydrodesulfurization (HDS) and hydrodemicrocarbon residue (HDMCR) or microcarbon reduction activity while maintaining catalyst sediment control functionality. Converting VR into lighter product occurs by thermocracking and catalytic hydrocracking at elevated temperature. Thus, suitable hydroprocessing catalysts are needed which provide suitable pore volume, surface area and pore size and distribution characteristics for maximizing catalytic hydrocracking reactions especially involving large molecules present in residuum. However, as discussed above, due to the nature of resid feedstock, catalysts gradually lose pore volume due to deposition of metals such as Ni and V present in the feedstock as well as deposition of coke that may form at elevated temperature, thus causing deterioration in hydrocracking activity of the catalyst with the progression of the reaction, and also increasing the formation of sediment.
Embodiments of the present invention include methods for preparing catalyst carriers as well as hydroprocessing, HDS, hydrodenitrification (HDN), hydrodemetallation (HDM) and HDMCR and other catalysts prepared using the carrier, and to processes for hydrodesulfurizing, hydrodenitrogenizing or hydrodemicrocarbonizing a hydrocarbon feedstock using the aforementioned catalyst. More particularly, embodiments also relate to a method for the preparing a porous catalyst carrier and catalyst using such carrier having preferred and defined pore characteristics, including pore size and pore size distribution, and containing at least one metal and/or metal compound of Groups 6 (also referred to as Group VIB) and Groups 8, 9 and 10 (also referred to as Group VIII) of the Periodic Table of the Elements.
In the course of conducting hydroprocessing reactions, unconverted or partially converted large feedstock molecules can aggregate and precipitate from whole liquid product and form sediment which is a highly undesirable hydroprocessing process by-product that can foul downstream equipment, such as heat exchangers, separators and fractionators. It would be desirable to develop catalysts exhibiting improved catalytic activity and/or stability, particularly in EB processes that do not negatively impact sediment formation, thus allowing refiners employing EB hydroconversion units to produce more and lighter petroleum products and with improved economics.